ar X iv : a st ro - p h / 04 03 09 9 v 1 4 M ar 2 00 4 KH 15 D : A Spectroscopic Binary 1

We present the results of a high-resolution spectroscopic monitoring program of the eclipsing pre–main-sequence star KH 15D that reveal it to be a singleline spectroscopic binary. We find that the radial velocity is variable with an observed range of 10.7 km s. The best-fitting Keplerian model has an orbital period P = 48.38 days, which is nearly identical to the photometric period. Thus, we find the best explanation for the periodic dimming of KH 15D is that the binary motion carries the visible star alternately above and below the plane of a circumbinary disk, as recently proposed by Winn et al. (2004) and Chiang & Murray-Clay (2004). We show that the mass ratio expected from models of PMS evolution, together with the mass constraints for the visible star, restrict the orbital eccentricity to 0.68 ≤ e ≤ 0.80 and the mass function to 0.125 ≤ FM/ sin 3 i ≤ 0.5 M⊙.